Country fields in D365 are text fields. The reason they are text fields is that that’s how it works in Outlook, and since addresses for contacts in OUtlook synchronize with D365, they are text fields in D365.

Many companies that use CRM have replaced the country and state fields with option sets or lookup fields; however, there are some considerations:

1. Outlook synchronization: keep in mind that contacts tracked in Outlook will create contacts in D365, and contacts updated in D365 will synchronize to Outlook, if you remove the text field and replace it with an option set, contacts synchronizing with Outlook won’t show the country, and contacts tracked in Outlook will update the text field, resulting in a country not being displayed on the contact form in D365.
2. More addresses: Addresses in D365 don’t actually live on the account or contact. They live in the customer address entity, which is called “more addresses” in the record navigation bar. Just because you add a custom country option field to the contact form won’t make the same option set field be populated in the customer address record.

Probably the easiest solution is to add the country option set and have a process update the standard text field with the value selected. This gives you a standard list but ensures that the standard country field is populated so it is populated in the address table as well as the contacts synchronized to Outlook. This doesn’t give a complete answer for contacts tracked from Outlook—the country option set would still be blank for these contacts, but you could create a cleanup batch job that updates these contacts that don’t have a country selected in the country option set.